1. Field of the Invention
The invention relates to a hybrid vehicle including an engine and a motor or motor generator as driving force sources, and a control method for the hybrid vehicle. Particularly, the invention relates to a hybrid vehicle configured to be able to separate an engine from a driving force transmission system, and a control method for the hybrid vehicle.
2. Description of Related Art
A hybrid vehicle including a motor or a motor generator (hereinafter, these may be collectively referred to as motor) as a power source together with an engine has various advantages. For example, the hybrid vehicle is able to stop the engine (stop the engine at idle), is able to regenerate energy with the use of the motor during deceleration, and is able to operate the engine at an operating point at which energy efficiency is high. Particularly, by extending a time in which the hybrid vehicle travels with the use of the motor, the effect of improving the fuel economy of the vehicle as a whole increases. This is because the engine is stopped and no fuel is consumed in the case of motor running. In this case, in order to reduce a power loss resulting from rotation together with the engine, it is desirable to separate the engine from the motor that outputs driving force for propelling the vehicle or from a power transmission system that transmits the driving force to wheels. When such a clutch for separating the engine is provided, an operation mode in which the engine is stopped or driven is selected during motor running. The hybrid vehicle including the clutch for separating the engine is described in Japanese Patent Application Publication No. 08-295140 (JP 08-295140 A).
The configuration will be simply described. A generator is coupled to a first rotating element in a differential mechanism, such as a planetary gear mechanism, including three rotating elements, a second rotating element serves as an output element, and a third rotating element is coupled to braking means. The engine is coupled to the third rotating element via the clutch. Thus, with the configuration described in JP 08-295140 A, the differential mechanism functions as a speed reducer or a speed increaser when the third rotating element is fixed by the engine or the braking means, so it is possible to cause the generator coupled to the first rotating element to function as the motor and to transmit the torque to the output element. That is, the hybrid vehicle is able to travel by using power that is output from the generator and the motor. When a one-way clutch that is engaged to fix the third rotating element when torque in a direction to rotate the third rotating element in the reverse direction acts on the third rotating element is employed as the braking means, the third rotating element is allowed to be fixed not by the engine but by the one-way clutch during motor running, and the clutch is also allowed to be separated from the third rotating element. Therefore, it is possible to continue driving the engine or stop the engine. During engine running, that is, when the hybrid vehicle travels by transmitting the output torque of the engine as driving force, it is possible to control the rotation speed of the engine coupled to the third rotating element via the clutch by controlling the rotation speed of the generator coupled to the first rotating element. That is, the differential mechanism is allowed to function as a continuously variable transmission.
Japanese Patent Application Publication No. 2013-023024 (JP 2013-023024 A) describes a hybrid vehicle including a differential mechanism having one rotating element to which an engine is coupled and the other rotating element to which a motor generator is coupled. The hybrid vehicle is configured to crank the engine by using the output torque of the motor generator and to control the output torque of the motor generator such that the output torque acts on the differential mechanism as reaction force after the engine is started. Specifically, when a required driving force after the engine is started is relatively small, a target rotation speed of the engine is also low. Therefore, in order for the engine rotation speed to quickly follow the target rotation speed after an engine start, the output torque of the motor generator is configured to rapidly increase. When a required driving force after the engine is started is relatively large, the target rotation speed of the engine is also high. Therefore, the output torque of the motor generator is configured to slowly increase.
Japanese Patent Application Publication No. 2012-228961 (JP 2012-228961 A) describes a hybrid vehicle in which an engine and a transmission are coupled to each other via a clutch and a motor generator is coupled to an input shaft of the transmission. At the time of changing from motor running where the engine is stopped to engine running, the engine is configured to be cranked by engaging the clutch with a small rotation speed difference between an input-side rotating member and output-side rotating member of the clutch through upshifting of the transmission. Specifically, the output-side inertia torque of the clutch is generated through shifting, and an engagement pressure of the clutch is controlled on the basis of the inertia torque. JP 2012-228961 A describes that, by controlling in this way, it is possible to crank the engine and to suppress or prevent transmission of the inertia torque to drive wheels.